Usage of :- (colon dash) in bash

Question

What is the meaning of this style in bash?

${PUBLIC_INTERFACE:-eth0}

What is the purpose of :-?

Answer 1

:- is used in the ${parameter:-word} shell parameter expansion: if parameter is null or unset, it expands to the value of word, otherwise to the value of parameter.

Example:

$ str=
$ echo "${str:-default}"
default

This and the similar expansions using :=, :+ and :? all come in two flavours: with and without a colon. The difference is that the expansion with the colon kicks in for "null or unset", whereas without the colon, it's just "null".

Observe:

$ str=                      # Null, but not unset
$ echo "${str-default}"     # Expands to value of $str, the empty string

$ echo "${str:-default}"    # Expands to "default"
default

Where is this useful? A few examples:

• Default values

  • The editor invoked to edit the last command with fc is the result of the expansion ${FCEDIT:-${EDITOR:-vi}}: $FCEDIT if defined, or else $EDITOR if defined, or else vi.

  • A loop in a script that should read from a file if one is provided as an argument and from standard input otherwise could look like this:

    while IFS= read -r line; do
        # do something
    done < "${1:-/dev/stdin}"
    

• When using set -u

  set -u is a handy way to force cleaner scripting by having the script die when encountering an unset variable, as promoted by for example this article (not that I endorse everything in there¹). If we want to check if a certain variable has a value with [[ $var ]], the script now dies if var is unset, even though this might be legitimate.

  The way around this is using [[ ${var:-} ]] instead, and set -u won't complain. (This is basically using a default value again, but the substituted value is the empty string in this case.)

These expansions are not exclusive to Bash, by the way: the POSIX shell spec has them all, too.

---

^{1 See also BashFAQ/112, What are the advantages and disadvantages of using set -u (or set -o nounset)?}